RESUMO
Mildly-acidic MnO2-Zn batteries are considered as a promising alternative for large-scale energy storage systems for their low toxicity, high safety, and low cost. Though, the degradation of MnO2 with cycling still hinders the further development of the batteries. In this study, it is observed that the decrease in available capacity of MnO2 with charge and discharge is accompanied by a structural transformation with the emergence of ZnâMnâO phases. An electrodeposition test indicates that the ZnâMnâO phase is formed from a co-precipitation of Zn and Mn during the charge process. Further, the structural change of MnO2 is suppressed and its cycle stability is improved with the addition of TiOSO4 as a facile electrolyte additive. As a result, under a current of 1200 mA g-1, the MnO2 electrode still gives a capacity of 230 mAh g-1 for over 1500 cycles. Capacity retention is 75% after 10 000 cycles under a current rate of 4800 mA g-1. These findings provide fundamental insights on the degradation mechanism of MnO2 and a new strategy to improve the electrochemical performance of aqueous MnO2-Zn batteries.
RESUMO
A highly active surface can endow the electrocatalysts with extraordinary catalytic performances. However, it remains challenging to tailor the atomic packing characteristics and thus the physical and chemical characteristics of the electrocatalysts. Herein, penta-twinned Pd nanowires (NWs) with abundant high-energy atomic steps (i.e., stepped Pd) are synthesized by seeded synthesis on Pd NWs enclosed by (100) facets. Benefiting from the catalytically active atomic steps, such as [n(100) × m(111)] on the surface, the resultant stepped Pd NWs can work as an effective electrocatalyst for the ethanol oxidation reaction (EOR) and ethylene glycol oxidation reaction (EGOR), which are essential anode reactions in direct alcohol fuel cells (DAFCs). Compared with commercial Pd/C, the Pd nanowires bound by (100) facets and atomic steps both display enhanced catalytic activity and stability towards the EOR and EGOR. Importantly, the mass activities of the stepped Pd NWs toward the EOR and EGOR are 6.38 and 7.98 A mgPd-1, which are 3.1 and 2.6 times those of Pd NWs enclosed by (100) facets, respectively. Besides, our synthetic strategy also enables the formation of bimetallic Pd-Cu nanowires with abundant atomic steps. This work not only demonstrates a simple yet effective strategy to obtain mono- or bi-metallic nanowires with abundant atomic steps, but also highlights the significant role of atomic steps for boosting the activity of electrocatalysts.
